the darkmoon rabbit is a pet which drops off of a world boss that spawns on darkmoon island, also known as the darkmoon rabbit. because it only drops one pet, and only spawns a few times a month, the supply of pets is fairly limited. the pet itself is unique (i think it's the same polygons as the crude basic rabbit, but with a more detailed texture applied). consequently, the auction house prices have been bid to astronomic levels - i was amazed to see it listed for 80k on my server, but apparently 200k is not unheard of. wow.
now, if you're a pet collector who wants one of everything, then only you can decide how much a unique pet is worth to you. for a pet battler looking for an advantage, though, the value is easy to determine: same as any other rabbit.
for starters, don't be fooled by the fact that the tooltip is epic. this means nothing, and i really wish blizzard would stop pulling this nonsense. the battle pet is rare. it has the same stats as a rare level one alpine hare you can catch in dun murogh. it does have a couple of unique abilities - huge, sharp, teeth! and vicious streak - but these are sidegrades from the standard rabbit abilities. possibly better in certain circumstances, but also possibly worse. certainly not worth dropping a small fortune for.
so, if you want a unique pet model, and you understand that's all you're getting, then rock on. but don't get suckered into thinking that because it is hard to obtain or has a pretty tooltip that it is somehow "better" or "op" for battling. 'cause it ain't.